This invention relates to the deposition of aluminium layers or films on a thin substrate.
In a number of applications, silicon wafers require thick aluminium layers to be deposited on them. For example, when the device being formed from the wafer includes high powered transistors, thick aluminium layers may be necessary as contact layers in order to handle the very high current densities inherent in these devices.
Typically such devices have a vertical architecture with a source contact comprising one or more aluminium layers from 1-20 μm thickness. These are deposited onto the semiconductor device (up to e.g. MOSFET.IGBT Bipolar) imbedded on a full thickness wafer. Typically the drain contact is formed on the back of the wafer. However, as most of the thickness is not useful to the device performance, but rather contributes to series resistance which wastes power, the wafers are ground back from typically 720 μm to <200 μm thick prior to the deposition of the drain contact. Such thin wafers are fairly flexible and subject to considerable warpage or bowing when under the stress induced by the various deposited layers.
It is known that sputtered aluminium layers deposited at temperatures >˜20° C. are tensile due to the mismatch of thermal expansion of the film and underlying substrate. Thus an 8 μm sputter Al film typically has a stress of ˜60 MPa. Table 1 below sets out the bow that can be induced in wafers of various thicknesses.
TABLE 1Induced Wafer Bow for different thickness Wafersas calculated using Stoney's EquationWafer ThicknessStressWafer BowμmMPaμm72060154200601992100607669
It will be seen that for a 200 μm Si wafer a bow of approximately ˜2 mm can be induced. Such a bow makes the wafer difficult to process in subsequent steps.
It is known that stress can be reduced to nearly zero by sputtering the film at low temperature with the wafer clamped to a cooled electrostatic chuck. The relationship between stress and platen or chuck temperature for an 8 μm film when there is no RF bias on the platen is shown in FIG. 1. It is further known that stress can be made compressive through the addition of RF bias and this is illustrated in FIG. 2.